Many sensors require specific orientation and insertion depth of the sensor probe for accurate measurements. These requirements apply to optical flow meters based on laser-two-focus (L2F) particulate velocimetry and optical flow meters based on laser-two-beam (L2B) particulate velocimetry. In L2F particulate velocimetry, an optical flow meter probe measures the velocity of gas particles by measuring the time delay between light scattering occurrences in two active sheets that are perpendicular to the gas flow direction and separated by a fixed distance. In L2B particulate velocimetry, an optical flow meter probe measures the velocity of gas particles by sensing the scintillation of light beams caused by flow turbulence. Both L2F and L2B types of optical flow meters require that the optical probe be positioned within a system with a specific orientation in relation to the direction of gas flow. Ultrasonic flow meters have multiple probes that have extremely tight tolerance insertion-depth requirements.
Other sensors such as temperature and pressure sensors require periodic removal for replacement, cleaning, and troubleshooting. The removal and subsequent reinstallation of these sensors occurs during operations requiring a method to remove the probe while maintaining process isolation.